Across the nation, many are looking to Accountable Care Organizations (ACOs) to improve care and reduce costs for Medicare and privately insured patients. A number of states have similar goals for ACOs in Medicaid, but the unique circumstances facing the Medicaid population require substantial departures from the approaches currently used within Medicare and private ACOs. In early 2015, New Jersey will implement a 3-year demonstration to test the ACO concept within its Medicaid program. The proposed study will provide the first rigorous large- scale evaluation of Medicaid ACO outcomes. It will also rigorously assess key elements of ACOs performance measurement, which are essential for further development of accountable care strategies by Medicaid and other payers. The first project aim is to test the hypothesis that patients in Medicaid ACOs experience lower spending and/or improved quality relative to other Medicaid enrollees. Under this aim, difference-in-differences models will be applied to multiple years of statewide Medicaid claims and managed care encounter records. In a subaim, simulation models based on the claims/encounter data will be used to assess the accuracy and statistical reliability of alternativ methods for measuring savings associated with Medicaid ACO activities. Under the second project aim, in-depth qualitative interviews will be used to assess how ACOs are redesigning care, engaging with patients, and modifying their strategies with accumulated experience. The third project aim is to engage regularly with Medicaid officials and accountable care stakeholders in NJ and across the nation to directly inform decision making about accountable care policy and practice. Input on evaluation measures and simulation scenarios will be solicited from these groups via existing state and national collaborations at every stage of the project to maximize the practical relevance and rapid dissemination of project findings. Dissemination products will include early data reports to Medicaid ACOs, webinars, in- person briefings, policy oriented issue briefs, and peer-reviewed manuscripts.